As far as I can tell, there is no scheduling assistant within the Webex web scheduler. You'll have to use your corporate calendaring tools, e.g. Outlook with plug-ins (or @ webex macro for personal rooms) in order to be sure the timeslot is open.
Hi I may have got your question wrong. But Webex Desktop App or anything else in Webex, does not offer calendaring availability - this is the role of your calendaring application for free/busy information. E.g. Outlook/Exchange or Google Calendar. Thanks ashish